It is sometimes necessary or desirable to measure various parameters of blood, such as hematocrit, oxygen saturation, carboxyhemoglobin, pH, etc. These blood parameters can all be measured using optical techniques.
For example, to measure oxygen saturation of whole blood, red light at, for example, 660 nanometers (nm) and infrared light at, for example, 805 nm are directed at the blood. The reflectance at both wavelengths is measured and appropriately ratioed to provide a measurement of oxygen saturation.
The hematocrit in whole blood may be measured, for example, by directing infrared light at the blood and detecting the reflectance of the infrared light at two spaced detectors. The hematocrit can then be determined by using a ratio of the two detected light levels or the difference between the two detected light levels. Another way to measure hematocrit is to employ a pair of spaced light sources and a single detector.
All of the techniques described above rely upon measuring of different detected light levels. Unfortunately, when the absorbance of the light directed at the blood is substantial, very low light levels are available for detection, and this results in a poor signal-to-noise ratio. In addition, when a constant level of light intensity is directed at the whole blood, the detected light level rises to a maximum near the middle of the physiologic range of hematocrit and then falls off this peak with increasing hematocrit levels. This curve creates an indeterminate condition in that it is not possible to determine which side of the peak is being observed so that an accurate hematocrit measurement is not obtainable.
Heinemann U.S. Pat. No. 4,447,150 discloses a technique f or measurement of blood oxygen saturation which compensates for variations of hematocrit levels by assuring uniform depth of penetration of light into the blood being sampled. In this system, red and infrared light sources direct light toward a blood sample, and a single detector detects the light which is reflected or transmitted. Optical feedback from the detector is used to control the light emitted by one of the sources so that the light detected by the detector from such source is constant. The intensity of the light emitted from the second source is determined by a ratio between the current needed to drive the first source and the current needed to drive the second source. This latter technique does not establish the intensity of the second light source as accurately as may be desired. Also, this technique either suffers from inaccuracy resulting from inherent differences and drift between the light sources or it requires a matched set, which is more expensive to provide.